Direct-reduction plants for the production of sponge iron from iron ore have gained increasing importance in recent years because the direct reduction process eliminates the need for blast furnaces, can be carried out at relatively low cost with long-life equipment, and produces a relatively high-purity product which can be utilized in steel-making processes directly, i.e., without smelting and refining before being introduced into the steel-making process.
The process is also advantageous since it does not require the use of limestone or slag-generating materials in the reduction stage.
As described in U.S. Pat. No. 2,740,706, the reducing gas can be produced from the furnace exhaust gas which is recirculated from the head of the furnace to a gas generator in which a partial combustion of a hydrocarbon fuel is carried out. Generally the recirculation cycle includes a moisture or water remover (demister) to eliminate excess water vapor or condensed moisture from the recirculated exhaust gas before it enters into a converter in which the partial oxidation takes place. The reaction involves the transformation of a carbon-containing energy carrier and oxygen, with residual carbon dioxide in the recirculated exhaust gas, into a gas stream consisting predominantly of hydrogen and carbon monoxide.
In the event the fuel for this latter process contains sulfur, there is always the danger that sponge iron, produced by the process will be contaminated with substantial quantities of sulfur which may detrimentally effect the steel-making process and the quality of the steel produced therein. Of course it is possible to reduce the sulfur level in the hot reducing gas produced by the conventional process, e.g. by using sponge iron as an adsorber for the sulfur before it is introduced into the reducing furnace, but this has the disadvantage that it produces a sulfur-contaminated product and has thermodynamic disadvantages for the metallurgical process taken as a whole.
Because of these limitation it has not been possible heretofore to use high-sulfur hydrocarbon fuels and, generally speaking, the fuels have been the lower hydrocarbons such as methane or ethane (e.g. natural gas).